1. Field of the Invention
The present invention relates to the field of amplifiers circuits and in particular to the field of low power, complementary output stages for amplifier circuits.
2. Related Art
Certain conventional amplifier circuits such as operational amplifiers and various CMOS amplifiers comprise differential input stages having active load gain stages and complementary output stages. An ideal complementary output stage should provide large voltage output swings and large output currents with low quiescent power dissipation. However, these features impose conflicting requirements, and available output stages include tradeoffs among power consumption, output voltage compliance, and output power.
For example, low power complementary output stages typically employ a class AB complementary source (emitter) follower configuration. In a complementary source (emitter) follower output stage, each transistor drives the load over only half the cycle, reducing the amount of quiescent current. The use of a complementary transistor pair increases the output voltage to approximately twice the value available from a single stage output circuit. However, the output voltage can only approach the supply voltage to within a gate-source (base-emitter) voltage drop. In addition, typical biasing schemes for the complementary source (emitter) follower transistors often do not provide sufficient voltages to the gates (bases) of the source (emitter) follower transistors, further limiting the output voltage.
Class A type output stages on the other hand provide high output power but at a cost of high quiescent power dissipation. In class A output stages, one transistor always remains fully turned on and the quiescent current must equal the maximum load current required. As with the complementary source (emitter) follower output stage, the output voltage and current may also be limited by the gate (base) voltage available to drive the output stage. There is thus a need for a complementary output stage capable of delivering high load currents and maximum output voltage swings with low quiescent power dissipation.